Bottom Line:
We first established a gastric lumen-perfusion system in the suncus and confirmed that intravenous (i.v.) administration of histamine (1 mg/kg body weight) stimulated acid secretion.Treatment with YM 022 (a CCK-B receptor antagonist) and atropine (a muscarinic acetylcholine receptor antagonist) had no effect on motilin or motilin-ghrelin co-administration-induced acid output.This is the first report demonstrating that motilin stimulates gastric secretion in mammals.

Affiliation: Area of Regulatory Biology, Division of Life Science, Graduate School of Science and Engineering, Saitama University, Saitama, Japan.

ABSTRACTMotilin and ghrelin constitute a peptide family, and these hormones are important for the regulation of gastrointestinal motility. In this study, we examined the effect of motilin and ghrelin on gastric acid secretion in anesthetized suncus (house musk shrew, Suncus murinus), a ghrelin- and motilin-producing mammal. We first established a gastric lumen-perfusion system in the suncus and confirmed that intravenous (i.v.) administration of histamine (1 mg/kg body weight) stimulated acid secretion. Motilin (0.1, 1.0, and 10 μg/kg BW) stimulated the acid output in a dose-dependent manner in suncus, whereas ghrelin (0.1, 1.0, and 10 μg/kg BW) alone did not induce acid output. Furthermore, in comparison with the vehicle administration, the co-administration of low-dose (1 μg/kg BW) motilin and ghrelin significantly stimulated gastric acid secretion, whereas either motilin (1 μg/kg BW) or ghrelin (1 μg/kg BW) alone did not significantly induce gastric acid secretion. This indicates an additive role of ghrelin in motilin-induced gastric acid secretion. We then investigated the pathways of motilin/motilin and ghrelin-stimulated acid secretion using receptor antagonists. Treatment with YM 022 (a CCK-B receptor antagonist) and atropine (a muscarinic acetylcholine receptor antagonist) had no effect on motilin or motilin-ghrelin co-administration-induced acid output. In contrast, famotidine (a histamine H2 receptor antagonist) completely inhibited motilin-stimulated acid secretion and co-administration of motilin and ghrelin induced gastric acid output. This is the first report demonstrating that motilin stimulates gastric secretion in mammals. Our results also suggest that motilin and co-administration of motilin and ghrelin stimulate gastric acid secretion via the histamine-mediated pathway in suncus.

pone.0131554.g005: Working hypothesis of stimulation of acid secretion by motilin in suncus.In the duodenum, motilin is secreted by the MO cell during the interdigestive state. It was hypothesized that motilin may directly stimulate histamine release through GPR38 expressed on the ECL cell. Another possibility is the indirect pathway, which is either central or peripheral neural stimulation of motilin for histamine release. Finally, the histamine induced by motilin acts on the parietal cell through the H2 receptor to release gastric acid. CCKB and muscarinic cholinergic receptors were not found to be involved in motilin-induced gastric acid secretion. MO cell: motilin-producing cell; GPR38: G-protein coupled receptor 38; CCK: cholecystokinin; CCKBR: cholecystokinin B receptor; ECL cell: enterochromaffin-like cell; ACh: Acetylcholine.

Mentions:
In this study, the effect of the co-administration of low-dose (1 μg/kg) motilin and ghrelin, but not motilin (1 μg/kg) or ghrelin (1 μg/kg) alone, was found to significantly stimulate gastric acid secretion compared with the vehicle and was almost comparable with that of 10 μg/kg motilin alone. This suggests that the additive role of ghrelin in motilin-induced gastric acid secretion is close to the physiological concentration. Moreover, further studies are required to elucidate the detailed mechanism of ghrelin, especially the target site of ghrelin. Meanwhile, we demonstrated the mechanism of action of motilin on gastric acid secretion. As summarized in Fig 5, we observed that gastric acid secretion by motilin administration was completely eliminated by pretreatment with famotidine (an H2 receptor antagonist), but not atropine or gastric/CCK-B receptor antagonists, indicating that motilin-stimulated acid secretion was finally mediated by the H2 receptor in the stomach. In addition, the suncus genome database confirmed that the predicted mRNA sequence of the H2 receptor, which is highly conserved in mammals, shows high homology with that in humans (data not shown). This suggests that histamine and famotidine are a specific agonist and antagonist, respectively, for H2 receptor in suncus. Hormone stimulating gastric acid secretion through histaminergic neural pathway is not a rare occurrence; for example, famotidine can inhibit ghrelin-stimulated gastric acid secretion in the rat [2] and PACAP (pituitary adenylate cyclase activating polypeptide) has also induced histamine release via a PACAP type 1 receptor on enterochromaffin-like (ECL) cells [3]. Motilin also stimulates gastric acid secretion via a histaminergic pathway, as was found in the case of ghrelin and PACAP. However, details regarding the specific target site of motilin for histamine release is unknown to date. There are three possible pathways that can be considered. One candidate is the central pathway. This possibility is supported by our findings that GPR38 mRNA is expressed in the nodose ganglion and the medulla oblongata [40], suggesting motilin affects vagus afferents and may stimulate acid output at its cephalic phase. Peripheral neural stimulation is the second candidate, as we showed that motilin excited cholinergic neural pathways in the myenteric plexus in an in vitro study of suncus; this enteric neural pathway may affect ECL cells. As a third candidate, the direct action of motilin on ECL cells cannot be ruled out. Further studies are needed for complete understanding of the target site of motilin, as well as the precise mechanism of motilin-induced gastric acid secretion.

pone.0131554.g005: Working hypothesis of stimulation of acid secretion by motilin in suncus.In the duodenum, motilin is secreted by the MO cell during the interdigestive state. It was hypothesized that motilin may directly stimulate histamine release through GPR38 expressed on the ECL cell. Another possibility is the indirect pathway, which is either central or peripheral neural stimulation of motilin for histamine release. Finally, the histamine induced by motilin acts on the parietal cell through the H2 receptor to release gastric acid. CCKB and muscarinic cholinergic receptors were not found to be involved in motilin-induced gastric acid secretion. MO cell: motilin-producing cell; GPR38: G-protein coupled receptor 38; CCK: cholecystokinin; CCKBR: cholecystokinin B receptor; ECL cell: enterochromaffin-like cell; ACh: Acetylcholine.

Mentions:
In this study, the effect of the co-administration of low-dose (1 μg/kg) motilin and ghrelin, but not motilin (1 μg/kg) or ghrelin (1 μg/kg) alone, was found to significantly stimulate gastric acid secretion compared with the vehicle and was almost comparable with that of 10 μg/kg motilin alone. This suggests that the additive role of ghrelin in motilin-induced gastric acid secretion is close to the physiological concentration. Moreover, further studies are required to elucidate the detailed mechanism of ghrelin, especially the target site of ghrelin. Meanwhile, we demonstrated the mechanism of action of motilin on gastric acid secretion. As summarized in Fig 5, we observed that gastric acid secretion by motilin administration was completely eliminated by pretreatment with famotidine (an H2 receptor antagonist), but not atropine or gastric/CCK-B receptor antagonists, indicating that motilin-stimulated acid secretion was finally mediated by the H2 receptor in the stomach. In addition, the suncus genome database confirmed that the predicted mRNA sequence of the H2 receptor, which is highly conserved in mammals, shows high homology with that in humans (data not shown). This suggests that histamine and famotidine are a specific agonist and antagonist, respectively, for H2 receptor in suncus. Hormone stimulating gastric acid secretion through histaminergic neural pathway is not a rare occurrence; for example, famotidine can inhibit ghrelin-stimulated gastric acid secretion in the rat [2] and PACAP (pituitary adenylate cyclase activating polypeptide) has also induced histamine release via a PACAP type 1 receptor on enterochromaffin-like (ECL) cells [3]. Motilin also stimulates gastric acid secretion via a histaminergic pathway, as was found in the case of ghrelin and PACAP. However, details regarding the specific target site of motilin for histamine release is unknown to date. There are three possible pathways that can be considered. One candidate is the central pathway. This possibility is supported by our findings that GPR38 mRNA is expressed in the nodose ganglion and the medulla oblongata [40], suggesting motilin affects vagus afferents and may stimulate acid output at its cephalic phase. Peripheral neural stimulation is the second candidate, as we showed that motilin excited cholinergic neural pathways in the myenteric plexus in an in vitro study of suncus; this enteric neural pathway may affect ECL cells. As a third candidate, the direct action of motilin on ECL cells cannot be ruled out. Further studies are needed for complete understanding of the target site of motilin, as well as the precise mechanism of motilin-induced gastric acid secretion.

Bottom Line:
We first established a gastric lumen-perfusion system in the suncus and confirmed that intravenous (i.v.) administration of histamine (1 mg/kg body weight) stimulated acid secretion.Treatment with YM 022 (a CCK-B receptor antagonist) and atropine (a muscarinic acetylcholine receptor antagonist) had no effect on motilin or motilin-ghrelin co-administration-induced acid output.This is the first report demonstrating that motilin stimulates gastric secretion in mammals.

Affiliation:
Area of Regulatory Biology, Division of Life Science, Graduate School of Science and Engineering, Saitama University, Saitama, Japan.

ABSTRACTMotilin and ghrelin constitute a peptide family, and these hormones are important for the regulation of gastrointestinal motility. In this study, we examined the effect of motilin and ghrelin on gastric acid secretion in anesthetized suncus (house musk shrew, Suncus murinus), a ghrelin- and motilin-producing mammal. We first established a gastric lumen-perfusion system in the suncus and confirmed that intravenous (i.v.) administration of histamine (1 mg/kg body weight) stimulated acid secretion. Motilin (0.1, 1.0, and 10 μg/kg BW) stimulated the acid output in a dose-dependent manner in suncus, whereas ghrelin (0.1, 1.0, and 10 μg/kg BW) alone did not induce acid output. Furthermore, in comparison with the vehicle administration, the co-administration of low-dose (1 μg/kg BW) motilin and ghrelin significantly stimulated gastric acid secretion, whereas either motilin (1 μg/kg BW) or ghrelin (1 μg/kg BW) alone did not significantly induce gastric acid secretion. This indicates an additive role of ghrelin in motilin-induced gastric acid secretion. We then investigated the pathways of motilin/motilin and ghrelin-stimulated acid secretion using receptor antagonists. Treatment with YM 022 (a CCK-B receptor antagonist) and atropine (a muscarinic acetylcholine receptor antagonist) had no effect on motilin or motilin-ghrelin co-administration-induced acid output. In contrast, famotidine (a histamine H2 receptor antagonist) completely inhibited motilin-stimulated acid secretion and co-administration of motilin and ghrelin induced gastric acid output. This is the first report demonstrating that motilin stimulates gastric secretion in mammals. Our results also suggest that motilin and co-administration of motilin and ghrelin stimulate gastric acid secretion via the histamine-mediated pathway in suncus.